1. Field of the Invention
This invention relates to the production of a magnetic material, and more specifically, to a process for producing a magnetic material for use in a magnetic recording medium suitable for high density recording.
2. Description of the Prior Art
According to the prior art, .gamma.-Fe.sub.2 O.sub.3, Co-containing .gamma.-Fe.sub.2 O.sub.3, Fe.sub.3 O.sub.4, Co-containing Fe.sub.3 O.sub.4, and CrO.sub.2 have been used as ferromagnetic powders for use in magnetic recording media. However, these ferromagnetic powders are not suitable for magnetic recording of signals of short wavelengths, e.g., wavelengths less than about 2 microns), and thus, have magnetic characteristics such as coercive force (Hc) and residual magnetic flux density (Br) which are insufficient for use in high density recording. In recent years, a number of various attempts to develop ferromagnetic powders having characteristics suitable for high density recording have been made. One material which has been investigated is a ferromagnetic metal powder, mainly of iron, cobalt or nickel with or without chromium, manganese, zinc, or rare earth elements.
Various methods are known for producing powdery metallic ferromagnetic materials. For example, the following six methods are known.
1. The reduction of an oxalate salt of a metal capable of forming a ferromagnetic material, at high temperatures in a stream of hydrogen (as disclosed, for example, in Japanese Patent Publications Nos. 11412/61, 22230/61, 14809/63, 8027/65, 14818/66, 22394/68, 38417/72 and 292280/73, and The Record of Electrical and Communication Engineering Conversazione Tohoku University, Vol. 33, No. 2, page 57, published 1964).
2. The reduction of needle-like oxyhydroxides with or without other metals or needle-like iron oxide obtained therefrom (as disclosed, for example, in Japanese Pat. Publications Nos. 3862/60, 20939/64 and 39477/72, Japanese Pat. application (OPI) No. 7153/71, German Pat. application (OLS) No. 2,130,921, British Pat. No. 1,192,167, and U.S. Pat. No. 3,681,018).
3. The vaporization of ferromagnetic metals in inert gases (as disclosed, for example, in Japanese Pat. Publication No. 27718/72, Japanese Pat. applications (OPI) Nos. 25662/73, 25663/73, 25664/73, 25665/73, and 55400/73, and Ohyo Butsuri (Applied Physics), Vol. 40, No. 1, page 110, (1971)).
4. The decomposition of metal carbonyl compounds (as disclosed, for example, in Japanese Pat. Publications Nos. 128/63 and 3415/65, and U.S. Pat. Nos. 2,983,997, 3,172,776, 3,200,007 and 3,228,882).
5. The electrodeposition of ferromagnetic metals in mercury using a mercury cathode, followed by separating the metals from the mercury by heating (as disclosed, for example, in Japanese Pat. Publications Nos. 787/64, 15525/64, and 8123/65 and U.S. Pat. No. 3,156,650).
6. The reduction of salts of metals capable of forming ferromagnetic materials in aqueous solutions thereof with a reducing substance (e.g., boron hydride compounds, hypophosphorous acid salts, or hydrazines) to form ferromagnetic powders (as disclosed, for example, in Japanese Pat. Publications Nos. 20520/63, 26555/63, 4567/66, 4769/66, 20116/68, 16052/72, 41718/72 and 41719/72, Japanese Pat. application (OPI) Nos. 1353/72 and 79754/73, U.S. Pat. Nos. 3,663,318, 3,661,556, 3,494,760, 3,206,338, 3,567,525, 3,535,104, 3,607,218, 3,669,643, 3,672,867 and 3,756,866, and German Pat. application OLS No. 2,132,430).
The present invention pertains to the method for reducing salts of ferromagnetic metals in solutions thereof described in paragraph (6) above, in which a boron hydride compound or a derivative thereof is used as the reducing agent.
The conventional method for preparing ferromagnetic powders by mixing an aqueous solution of a salt of a ferromagnetic metal with a reducing agent containing a boron hydride compound or a derivative thereof presents the following problems.
1. The coercive force of the ferromagnetic powder formed can be adjusted within the range of about 10 to 2,000 Oe. The method, however, has poor reaction efficiency when it is desired to obtain powders having a high coercive force of at least 1,000 Oe, particularly 1,200 Oe or more and a good squareness ratio. The reaction must be carried out in a bath having a very low concentration, and the method is uneconomical for mass production.
2. It is difficult to obtain powders consisting mainly of Fe or Co and additives for improving the stability to oxidation, etc., such as rare earth elements, elements of group IVa, elements of group VIa, elements of group VIIa, elements of group VIII, elements of group Ib, elements of group IIb, elements of group IIIb, elements of group IVb, and elements of group Vb which have a high coercive force of more than 1,200 Oe and good squareness ratio.
On the other hand, as a magnetic material having a high coercive force of 1,000 Oe or more, iron oxides containing Co are well-known. These iron oxides, however, have the defects that the thermal demagnetization and the heat demagnetization are high and the Hc and the squareness ratio thereof are decreased at a high temperature. Furthermore, it was found that the Bm of a magnetic recording medium using an iron oxide is lower in comparison with that of a magnetic recording medium using a ferromagnetic metal alloy powder, e.g., in using as a master tape for magnetic duplication by contact print, an out-put of a slave tape duplicated using an iron oxide is lower by 4 to 6 dB than that using a ferromagnetic metal alloy.
It has now been found that the coercive force of a metal powder for a magnetic recording medium can be increased by treating the powder with an inorganic acid such as sulfuric acid, etc. However, because of a decrease in the saturation magnetization (.alpha.s), a decrease in the oxidation resistance, and an increase in the half width of the differential curve of the hysteresis curve, it has been found that use of a metal oxide for a magnetic recording medium is disadvantageous in practice due to the decrease in Bm and in the squareness ratio (Br/Bm).